Contrasting partition behavior of F and Cl during hydrous mantle melting: implications for Cl/F signature in arc magmas

We present the results of five experiments on F and Cl partitioning during hydrous mantle melting under conditions relevant to subduction zone magmatism (1.2-2.5 GPa, 1,180 degrees C-1,430 degrees C). For each experiment, we determined the F and Cl partition coefficients between lherzolitic mineral phases (olivine, orthopyroxene (opx), clinopyroxene (cpx), and garnet), amphibole, and hydrous basaltic melts (0.2-5.9 wt.% dissolved H2O). At constant pressure, D-F(opx/melt) and D-Cl(opx/melt) show contrasting response to the combined effects of decreasing temperature from 1,310 degrees C to 1,180 degrees C and increasing H2O content in the melt from 0.2 to 5.9 wt.%: D-F(opx/melt) . decreases from 0.123 +/- 0.004 to 0.021 +/- 0.014 while D-F(opx/melt) increases from 0.0021 +/- 0.0031 to 0.07 +/- 0.01. Similar results are observed for clinopyroxene: D-F(cpx/melt) decreases from 0.153 +/- 0.004 to 0.083 +/- 0.004 while D-Cl(cpx/melt) increases from 0.009 +/- 0.0005 to 0.015 +/- 0.0008. Experimentally determined F and Cl partition coefficients were used in a hydrous melting model of a lherzolitic mantle metasomatized by slab fluid. In this model, we vary the amount of metasomatic slab fluid added into the mantle while its composition is kept constant. Increasing the amount of fluid results in an increase of both the degree of melting (due to the effect of H2O addition) and the F and Cl input in the mantle wedge. Because of the change of F and Cl partition coefficients with the increase of H2O, the observed variation in the F and Cl contents of the modeled melts is produced not only by F and Cl input from the fluid, but also by the changes in F and Cl fractionation during hydrous melting. Overall, the model predicts that the Cl/F ratio of modeled melts increases with increasing fluid fraction. Therefore, a variation in the amount of fluid added to the mantle wedge can contribute to the variability in Cl/F ratios observed in arc melt inclusions.